This invention relates to multi-stage polymer particles containing void space(s) therein which are useful as binding or opacifying agents in coating, impregnating and molding compositions.
Core-shell polymer particles are disclosed in U.S. Pat. No. 4,427,836. The polymer particles of the '836 patent are prepared by sequentially emulsion polymerizing a core monomer system comprising monoethylenically unsaturated monomers, one of which has a carboxylic acid group and said carboxylic acid group containing monomer comprising at least 5% by weight of the core monomer system, polymerizing in the presence of the core polymer dispersion a shell monomer system which is permeable to aqueous volatile base selected from ammonia and amines, and neutralizing with ammonia or amine so as to swell said core and form particles which, when dried, contain a single void and cause opacity in compositions in which they are contained. The '836 patent requires that the carboxylic acid be present in the core in a polymeric form. The composition of the shell polymer is such as to be not permeable to fixed or permanent base according to said patent "so that films deposited from aqueous coating compositions comprising a volatile base-swollen core of the core/sheath polymer upon drying and resultant (at least partial) removal by volatilization of the base are not damaged by any permanent base present in the substrate coated or in solutions used later for cleaning the films." (Column 3, lines 35-41). Said patent teaches that "suitable swelling agents for acid-containing cores are ammonia, ammonium hydroxide, or a volatile lower aliphatic amine, such as trimethylamine, and triethylamine."
Similarly, U.S. Pat. No. 4,468,498 discloses core-shell polymers having a core of polymeric acid and being swellable by a volatile base.
U.S. Pat. No. 4,594,363 discloses a process for making core-shell polymer particles similar to the '836 patent, except that a fixed or permanent base is used to swell the particles. The '363 patent also requires that the core be polymerized from at least 5% by weight of acidfunctional monomer.
U.S. Pat. No. 4,469,825 discloses core-shell polymers having cores which are swellable by acid.
High levels of copolymerized acid in the core polymer result in several disadvantages. One such disadvantage is that the acidic copolymers are polar and do not adsorb surfactants well, thus making them unstable and prone to coagulation or aggregation into larger particle. Another disadvantage is that the acidic core polymers have a high affinity for water. This affinity makes it difficult to encapsulate the core with the shell polymer because the core tends to remain at the polymer/water interface on the surface of the composite particle.
The present invention eliminates the instability, coagulation and encapsulation problems of polymers having relatively high levels of copolymerized acid in the core. Applicants have discovered that swellable, multi-stage polymer particles can be prepared by introducing a non-polymeric carboxylic acid to an earlier stage of the particles. By introducing a non-polymeric acid to an earlier stage after polymerization of said earlier stage, (preferably after polymerization of the complete, multi-stage polymer) applicants have overcome the disadvantages of the core-shell polymers containing copolymerized acid in the core.
It is an object of the present invention to provide opacifying, multi-stage polymer particles having a non-polymeric carboxylic acid absorbed in an earlier stage of the particles. It is a further object of this invention to provide a process for making multi-stage polymer particles wherein the earlier-stage particles are relatively stable and easy to encapsulate.